In-rack cooling systems provide highly focused cooling solutions by placing the cooling unit directly in or on an equipment rack. It may be a closed-loop system built in a special enclosure that doesn’t depend on room cooling at all, or it may be a heat exchanger attached to the side or back of a rack or incorporated into the rack’s rear door. Most systems require that chilled water or a liquid refrigerant be installed in the data center and distributed to the racks.